Carrier grade networks are expected to provided a high degree of availability although of in the networks elements can fail. This challenge arises, e.g., for virtual private networks or in the terrestrial radio access network of the universal mobile telecommunication system (UMTS). In contrast Internet Protocol (IP) technology enables a global interconnection of computer controlled devices, e.g., hosts, servers and terminals, on a unreliable best effort basis.
In time division networks, e.g., telephone networks, the reliability is ensured by hardware redundancy of the different network elements. Packet-oriented networks, e.g., the IP network, are protected against link failure by backup links or by ring shaped networks on the physical layer, e.g., SDH rings. However, these methods require a backup capacity which comprises at least the capacity of the failed network elements.
In packed-switched networks, a high degree of reliability can be achieved by traffic deviation over alternative paths in case of local outages. The backup capacity may be shared among different traffic aggregates in different failure scenarios. Therefore, the required backup capacity can be reduced without compromising the failure resilience of the network.
For the routing of the packets in packet switched networks there are two different routing methods. The routes in a destination address based routing used in the internet protocol (IP) are usually set up by the routing protocol like the open shortest path first protocol and the destination address transmitted in protocol header. Load balancing over multiple paths is possible if several routes to the same destination reveal the same costs. The traffic in connection-oriented routing used in MPLS networks is forwarded along virtual connections whereby each virtual connection is assigned a label. The virtual connections including the label are established or set up before the forwarding of the packets and the routes can be chosen arbitrarily, e.g. using an explicit route object in MPLS.
In packet-switched networks like IP and MPLS networks, traffic is deviated over alternative paths in case of a local outage. There are basically two options for resilient mechanisms.
With local path restoration in case of MPLS networks or with local rerouting in case of IP technology, a deviation path or route is only activated if a failure of the local network element is indicated. I.e., in IP networks a link failure is detected and indicated by missing hello messages of the open shortest path first protocol (OSPF). Backup capacity can be shared because no resources are bound to any aggregate before the failure is indicated. However, the reaction time of restoration mechanism especially for networks with high transmission rates is to long.
To avoid long reaction time with path protection, the primary path failure is anticipated, i.e., a back up path is set up before a failure is indicated, whereby the traffic is transmitted simultaneously over the primary and the backup path. In this case a backup capacity sharing is not possible.
A new approach for routing in packet-switched networks is multi-path routing. In the source network element, the traffic is distributed over the several paths wherein the paths are established connection-oriented. The distributed traffic is transmitted parallel over the paths and reassembled at the destination network element. If an outage of one or more network elements is indicated, i.e., if an inactive condition of the local network elements is indicated, the traffic is deflected to the remaining active paths.